In a conjugated diene-based polymer obtained by copolymerizing one or more conjugated dienes or one or more conjugated dienes and a vinyl aromatic compound using an organic alkali metal compound as a polymerization initiator, it is known that its heat resistance, oxidation resistance, weather resistance, or ozone resistance, or the like can be improved by hydrogenating a carbon-carbon double bond based on a conjugated diene structural unit of the polymer, and that a a hydrogenated product of such a conjugated diene-based polymer is industrially useful as elastic bodies or thermoplastic elastomers.
As a hydrogenation catalyst on the occasion of hydrogenating a conjugated diene-based polymer, nickel-based or cobalt-based Ziegler-Natta catalysts and so on have hitherto been suitably used. However, in order to suppress coloration to be caused due to the residues of a component derived from the catalyst in the resulting hydrogenated product, it was necessary to remove the catalyst residues derived from such a Ziegler-Natta catalyst from the hydrogenation reaction liquid by a means, such as extraction, washing, etc., prior to separation and acquisition of the hydrogenated product. Meanwhile, if a titanium-based catalyst, especially a titanocene-based compound that is a metallocene-based compound, is used as a hydrogenation reaction catalyst of a conjugated diene-based polymer, it has higher catalytic activity than the Ziegler-type catalyst, and it is possible to undergo the hydrogenation reaction using a smaller amount of the catalyst. Therefore, an operation of removing the catalyst component from the hydrogenation reaction liquid becomes unnecessary, or even when the operation is performed, a means of removing the catalyst component may become simple and easy. In consequence, a hydrogenation reaction of a conjugated diene-based polymer using a titanocene-based compound as a catalyst is investigated (see PTLs 1 to 11).
Among the titanocene-based compounds, a solution obtained by allowing bis(cyclopentadienyl)titanium dichloride to react with two equivalents of trimethylaluminum in a toluene solvent is called a Tebbe reagent, and μ-chloro-μ-methylene-bis(η5-cyclopentadienyl)titaniumdimethylaluminum (Cp2TiCH2AlClMe2) that is mainly existent is known as a Tebbe complex. In addition, the Tebbe complex can be isolated from the Tebbe reagent through a recrystallization operation (see NPLs 1 to 3). It is known that a Tebbe-type metallacycle compound represented by the Tebbe complex is useful as a hydrogenation catalyst of a carbon-carbon double bond based on a conjugated diene structural unit of a conjugated diene-based polymer (see PTLs 2 to 3 and 6 to 8).
More specifically, PTLs 2 and 3 disclose a method in which at least one conjugated diene is polymerized or copolymerized using an organic alkali metal compound as a polymerization initiator in the presence of a solvent and then allowed to act on hydrogen to terminate the polymerization, and the resulting conjugated diene-based polymer is allowed to react with hydrogen in the presence of an accelerator that is the organic alkali metal compound and a Tebbe complex, thereby selectively hydrogenating an unsaturated double bond in the conjugated diene structural unit, and describe that in order to achieve a hydrogenation degree of 95% or more, a ratio of the alkali metal atom to the titanium atom is at least 2 or more, and preferably 5 to 15. In addition, in the case where the solution of the conjugated diene-based polymer is high in viscosity (the conjugated diene-based polymer has a high molecular weight), it is necessary to increase the ratio of the alkali metal atom to the titanium atom. It is disclosed that if an alkali metal hydride must be added in order to increase this ratio, after adding the organic alkali metal compound in the system of before and after the polymerization termination reaction, by allowing the resultant to act on hydrogen dispersed by spargers, the alkali metal hydride can be prepared within the system.
PTLs 4 and 5 disclose the hydrogenation reaction using a titanocene compound that is different from the Tebbe-type metallacycle compound and making lithium hydride coexistent.
In detail, PTL 4 discloses a method in which (1) at least one conjugated diene is homopolymerized or copolymerized using an organic lithium compound as an initiator to prepare a living polymer; (2) the formed living polymer is end-terminated using an equimolar amount of an end-modifying material; and (3) a specified monocyclopentadienyl titanium compound and lithium hydride as prepared from an organic lithium compound and hydrogen are added to the aforementioned end-terminated polymer individually or in a mixed form by premixing outside, and the conjugated diene-containing polymer is then selectively hydrogenated.
PTL 5 discloses a method for hydrogenation of a conjugated diene polymer, the process including the steps of (a) polymerizing or copolymerizing at least one conjugated diene in a hydrocarbon solvent using an organic alkali metal polymerization initiator to form a living polymer; (b) adding one or more end-modifying agent selected from the group consisting of amines, alcohols, esters, ketones, and halogen compounds to deactivate an active terminal of the living polymer to form a conjugated diene polymer; and (c) selectively hydrogenating an unsaturated double bond of a conjugated diene unit of the conjugated diene polymer using a specified cyclopentadienyltitanium compound and a highly active lithium hydride obtained by precisely controlling a particle diameter by a reactor equipped with a high-speed injection nozzle.
NPL 4 reports that in the hydrogenation reaction of a terminal alkene, such as 1-hexene, etc., with a titanocene compound using sodium hydride as a cocatalyst, the prepared sodium hydride of a nanometer size (specific surface area: 90 m2/g) exhibited high hydrogenation activity; however, in the case of using commercially available sodium hydride (specific surface area: 1.4 m2/g), the hydrogenation reaction does not proceed at all.
PTL 6 discloses a hydrogenation method of a conjugated diene polymer, in which on the occasion of adding a deactivating agent (the molar amount of the deactivating agent is defined as Z) to a conjugated diene polymer obtained through polymerization with, as a polymerization initiator, an organic alkali metal compound (the molar amount of the alkali metal compound contained is defined as M) to achieve deactivation and bringing the resultant into contact with hydrogen in an inert hydrocarbon solvent to hydrogenate a double bond of the conjugated diene unit, the hydrogenation is performed in the presence of an organic titanium compound that is a Tebbe-type metallacycle compound (the molar amount of the organic titanium compound is defined as Ti, and the molar amount of an organic aluminum compound is defined as Al) within a range of (−6≤(M-Z+Al-Ti)/Ti≤+2). Here, Ti is corresponding to a total molar amount of an unreacted material of the organic titanium compound that is a synthetic raw material of the Tebbe-type metallacycle compound, the Tebbe-type metallacycle compound, and by-produced other organic titanium compounds, and Al is corresponding to a total molar amount of an unreacted material of the organic aluminum compound that is a synthetic raw material of the Tebbe-type metallacycle compound, aluminum existent in the Tebbe-type metallacycle compound, and aluminum in by-produced other organic titanium compounds.
PTL 7 discloses a hydrogenation method of a conjugated diene-based polymer, in which on the occasion of hydrogenating a conjugated diene-based polymer obtained through polymerization with an organic alkali metal compound as a polymerization initiator by using a metallocene-based hydrogenation catalyst to obtain a conjugated diene-based polymer having a hydrogenation rate of 98% or more, the hydrogenation catalyst is added dividedly two or more times, and preferably, at the point of time when the hydrogenation rate reaches 60% to 95%, the hydrogenation catalyst is added one or more times, thereby advancing the hydrogenation. In addition, it is also disclosed that the timing of addition of the hydrogenation catalyst is determined by measuring an absorption rate of hydrogen (see PTL 8). Here, as an example of the metallocene-based hydrogenation catalyst, a Tebbe-type metallacycle compound is exemplified.
PTL 9 discloses a catalyst composition composed of at least one compound among an oxygen-containing organic compound or nitrogen-containing organic compound having two or more carbon atoms and a salt thereof, and a Tebbe-type metallacycle compound and discloses that when an olefinic unsaturated double bond of an olefin compound, particularly a conjugated diene-based polymer is hydrogenated using the foregoing catalyst composition, even if an alkyl alkali metal compound as a cocatalyst is not used, sufficient hydrogenation catalytic activity is exhibited in a use amount at a level of not requiring decalcification, and excellent heat resistance of the catalyst is revealed. It is also disclosed that by properly further combining specified other organic metallic compounds, the long-term storage stability is improved, thereby enabling the activity stability to be kept over a long period of time.
In addition, PTLs 10 and 11 disclose the hydrogenation reaction of a conjugated diene-based polymer having high hydrogenation activity and excellent stability of catalyst (heat resistance and storage stability) in the presence of a titanocene compound that is different from a Tebbe-type metallacycle compound, a specified silyl hydride compound, and, as a third component, an alkali metal hydride, an alkali metal alkoxide, an organic aluminum compound, an organic magnesium compound, an organic zinc compound, an organic titanium compound other than a titanocene compound, or the like.